Lignin is the main source of color in pulp due to the presence of a variety of chromophores naturally present in the wood or created in the pulp mill. Chemical pulps contain very little lignin while mechanical pulps contain most of the lignin that was present in the wood used to make the pulp. While the results are the same, the processes and fundamental chemistry involved in bleaching chemical pulps (like kraft or sulfite) are very different from those involved in bleaching mechanical pulps (like stoneground, thermomechanical or chemo-thermomechanical). Writing and printer paper would typically be as bright as 104 ISO. Newsprint ranges from 55-75 ISO brightness. Table 1: Comparison of the US and ISO systems Because the ISO standard only measures a narrow range of blue light, it is not directly comparable to human vision of whiteness or brightness. This is because contemporary white paper incorporates fluorescent whitening agents (FWA). The ISO rating is higher and can be over 100. Table 1 shows how the two systems rate high brightness papers, but there is no simple way to convert between the two systems because the test methods are so different. The international community uses ISO standards. In the US, the TAPPI T 452 or T 525 standards are used. Brightness is the amount of incident light reflected from paper under specified conditions, usually reported as the percentage of light reflected, so a higher number means a brighter or whiter paper.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |